Not Applicable
Not Applicable
The present invention relates to, a patient-shielding system for use when a patient is exposed to capacitive currents as a result of immersion into a time-varying magnetic field. More particularly, this invention relates to a system for redirecting potentially harmful currents away from organs such as the heart when a medical procedure includes exposing that organ to a time-varying magnetic field.
Systems and methods for determining the position and orientation of surgical probes based on the use of magnetic fields are known. See, for example, U.S. Pat. No. 5,592,939. Such systems and methods generally rely on the presence of a time varying magnetic field in the surgical region of interest. An exemplary navigation system is shown in FIG. 1. The exemplary system of FIG. 1 contains platform 10 in which is embedded coils for generating a time-varying magnetic field. Two such coils are depicted as first coil set 12 and second coil set 14. Field line 22 depicts the orientation of a magnetic field amplitude at an instant of time. See also U.S. Pat. No. 5,592,939.
Present techniques for projecting a time varying magnetic field into a surgical region of interest preferably position the patient proximal to the coils that are generating the necessary fields. This is depicted in FIG. 2. Patient 24 is generally kept from direct contact with coil sets 12 and 14 by non-conducting layer 20. As a result of this relationship, there are times when coil sets 12 and 14, located proximally to the surgical region of interest, may have differing voltage potentials. By way of example only, in FIG. 2, coil set 12 is at positive potential 16, and coil set 14 is at negative potential 18. A uniform amplitude field that has its major component lateral to a plane determined by an operating room table is thus generated by two coils at different voltage potentials separated along that lateral dimension. Field line 22 in FIG.2 indicates the direction of such an amplitude. In the relationship indicated in FIG. 2, the surgical region of interest has loop characteristics of what is known as a capacitive current. A schematic of such a current is depicted in FIG. 3. For a time-varying magnetic field where the frequency is of the order of f=20 kilohertz and the difference between positive potential 16 and negative potential 18 is V=25 volts, capacitive current 34, denoted by I, can exceed what is considered desirable. For example, typical safety standards, such as those of Underwriter Laboratories, require that the current through a patient be less than I=10 microamps. For insulating layer 20 with capacitance 30 of the order of C=10xe2x88x9210 farads, and where patient 24 has a resistance 32 of approximately 100 ohms, capacitive current 34 is of the order
I=V(2xcfx80fC)=345 microamps
This is well in excess of a 10 microamp current.
In light of the foregoing, it is desirable to reduce the magnitude of the capacitive current introduced by a magnetic field coil within a surgical region. It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.
The foregoing and other objects are achieved by the invention which in one aspect comprises a patient-shielding and coil system, including a coil wire electrically coupled to a source of electrical current, an electrically conductive surface, insulation material situated between the coil wire and the conductive surface, and a drain wire connected to the conductive surface and forming a capacitive current loop with respect to the source.
In another embodiment of the invention, the conductive surface has a resistance of substantially 1 ohm per square.
In another embodiment of the invention, the electrically conductive surface forms an incomplete enclosure of the coil wire, so as to create an incomplete electrical circuit.
In another embodiment of the invention, the conductive surface includes an upper portion and a lower portion.
In another embodiment of the invention, the conductive surface includes a polyester foil, vapor deposited with aluminum.